1. Field of the Invention
This invention relates to a digital signal encoding apparatus for encoding digital signals, such as digital audio or voice signals.
2. Description of the Prior Art
In high efficiency encoding of audio or voice signals in the prior art, input audio or voice signals are divided on a time axis or on a frequency axis into a plurality of channels and the associated bits are adaptively allocated to the respective channels. Examples of encoding techniques of audio signals by such bit allocation include sub-band coding (SBC) in which the audio signals on a time axis are divided into a plurality of frequency bands for encoding, adaptive transform coding (ATC) in which signals on a time axis are converted into signals on a frequency axis by orthogonal conversion and divided into a plurality of frequency bands which are then adaptively encoded, and a so-called adaptive bit coding (APC-AB) in which the SBC and ATC encoding techniques are combined so that the signals on a time axis are divided into frequency bands whereupon the signals of the respective bands are converted into baseband signals (low-range signals) and predictively encoded by n'th order linear predictive analyses, where n is an integer equal to or greater than two.
In the above-described high efficiency techniques, adaptive encoding on a band-by-band basis is typically performed by calculating the energy or the in-band peak value from ne band to another and then quantizing the components in each band with the number of bits allocated in accordance with the thus calculated respective energy.
Recently, in encoding audio signals, there is an increasing demand towards higher bits compression which is more satisfactory to a human's auditory sense than that achieved with the above mentioned conventional high efficiency encoding. As a result, it has been desired to perform encoding with higher data compression or bit compression which is more acceptable to the human auditory sense which considers the characteristics of the input audio signals or the human auditory sense.
In general, audio signals are known to have a relatively lesser amount of energy in the higher frequency components. For this reason, input audio signals are processed by pre-emphasis in advance of the encoding so as to equivalently decrease the high range noise with respect to the signal components thereby improving the signal-to-noise (S/N) ratio. In this case, deemphasis is performed during signal reproduction.
Thus, in encoding the audio signals which are to be divided into frequency bands, pre-emphasis and deemphasis are performed with quantization in which, as previously described, the number of bits are adaptively allocated to each of the frequency bands. More specifically, digital audio signals are processed by pre-emphasis and subsequently divided into a plurality of frequency bands. The signal components of each band are then quantized with the number of bits allocated in accordance with the energy of the respective frequency band. In this situation, the arithmetic operation for finding the energy in each frequency band is performed on signals which have already been processed by pre-emphasis.
As is to be appreciated, the number of quantization bits allocated on the basis of the energy of signals already processed by pre-emphasis may not be equivalent to the number of bits allocated on the basis of the properties of the actual input audio signals preceding the pre-emphasis. Thus, there is a substantial risk that the decoded audio or voice signals which are reproduced will be perceived by a human's auditory sense as being different from the original inputted signals.